Thank you for your letter of November 6, 1991 requesting interpretations and
clarifications of 29 CFR 1910.106 standards on outside, above ground, tanks
used for storage of flammable and combustible liquids. Responses to your
questions follow. Please accept our apology for the delay in our response.

Question 1: Is the scope of 29 CFR 1910.106 intended to

be limited to the storage of non-reactive flammable and
combustible liquids at low pressure (less than 15 psig), or is this standard
applicable to these liquids regardless of system
pressure?

Reply: The standards at 29 CFR 1910.106 apply to stable
as well as unstable (reactive) combustible and flammable liquids as defined
in paragraph 1910.106(a). The standards at paragraph 1910.106(b) apply to
atmospheric tanks, low pressure tanks and pressure vessels, as defined in
paragraph 1910.106(a) for tank storage of combustible and flammable liquids
in the workplace. Pressure vessels which are designed, constructed,
installed, operated and maintained in compliance with paragraph 1910.106(b)
shall be used to store combustible and flammable liquids at (1.05 kilograms
per square centimeter) gauge (p.s.i.g.).

Question 2: If the scope of 29 CFR 1910.106 is not limited

to a given system pressure range, is the method for
sizing emergency relief devices for the storage of non-reactive materials at
pressures above 15 psig presented in API-520 acceptable to meet the
requirements of 29 CFR 1910.106? If API-520 is considered by OSHA to not
meet these requirements, then I recommend OSHA and industry experts meet to
discuss the issue and resolve the discrepancies.

Reply: Installation of emergency relief venting for
outside, above ground, tanks used to store combustible and flammable liquids
in the workplace shall comply with paragraph 1910.106(b)(2)(v). Use of
emergency relief venting predicated on sizing required to comply with
American Petroleum Institute (API) standard, API RP-520 in place of emergency
relief venting which complies with the capacity requirements of paragraph
1910.106(b)(2)(v) is subject to Occupational Safety and Health Administration
(OSHA) consideration as a variance. Should you choose to pursue a variance,
please refer to the enclosed copy of 29 CFR 1905. Section 1905.11 delineates
the criteria necessary to obtain a permanent variance. With respect to
paragraph 1910.106(b)(2)(v) requirements, you must demonstrate that the
emergency relief venting sized to comply with API RP-520 provides equal or
greater protection to employees from fire and explosion hazards in the
workplace. Your application should be forwarded to:

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